The invention relates to a measuring apparatus for fluids, it being understood that under xe2x80x9cfluidsxe2x80x9d, both a gas and also a liquid are encompassed.
In the state of the art, measuring apparatuses have been developed in which a rotor is maintained in an equilibrium position within a carrier tube by magnetic field forces. Thus in DE-A-29 19 236, a turbine wheel counter is described for the flow-through measurement of liquids in which the rotor has, for radial stability, two spaced apart rotor magnets formed as permanent magnets which are juxtaposed pairwise with stator magnets also formed as permanent magnets, which surround the carrier tube. Thus the rotor magnets and stator magnets are magnetized in the axial direction so as to repel one another.
Between the stator magnets, an electromagnet coil is arranged which surrounds the carrier tube in an annular manner. The magnet coil cooperates with a ferromagnetic flux conductive piece on the rotor which is arranged between the rotor magnets.
Additionally, a sensor is provided that detects the axial position of the rotor and cooperates with a control unit which regulates the electric current flow in the magnet coil. As soon as the field forces of the rotor magnets and the stator magnets are engendered by an axial shift of the rotor to accelerate the rotor out of the equilibrium position in the axial direction there is generated a signal of the axial shift of the rotor as measured by the sensor which produces in the magnet coil a counteracting stabilizing field force. The rotor is thus, upon an axial shift in position in one or the other direction, always returned to its setpoint position. The stabilizing axial force countering the axial shift is so phase-shifted in time in a known manner that the rotor is held stably in its setpoint position by restoring as well as damping forces.
A drawback of the aforedescribed measuring device is that the rotor has only a relatively limited positional stiffness in the radial direction. The origin of that problem is the large distance between the stator and rotor magnets because of the annular channel between the support pipe and the rotor for the throughflow of the fluid.
In DE-A-24 44 099, a magnetic bearing for rapidly movable bodies is described. This magnetic bearing has a sleeve-shaped rotor whose ends have pole pieces provided with permanent magnets and whereby, based upon tractive forces on the rotor, the latter is held in a stable position. By means of a contactless position sensor, deviations from the equilibrium position are determined. Such deviations are compared by a powerless electromagnetic stray field control for which annular coils are provided which are arranged proximal to the gaps on the pole pieces of the rotor. Such a magnetic bearing is not suitable for disposition in a carrier tube through which a fluid is conducted on spatial grounds.
Magnetic bearings have also been used for blood pumps. Thus in U.S. Pat. No. 5,695,471, a blood pump is known which is formed as a radial pump with a radial rotor. The radial rotor is disposed within a carrier tube and has in an inlet side extension, a multiplicity of rotor magnets which are juxtaposed with stator magnets on the carrier tube. Additionally the radial rotor has distributed over the periphery a multiplicity of rod-shaped rotor magnets extending in the axial direction and which are juxtaposed with ring-shaped stator magnets on both sides of the radial rotor toward the carrier tube. These rotor and stator magnets should support the radial journaling in the region of the extension of the rotor. In the axial direction, the rotor is held purely mechanically with one end on a ball and another end on a point journal. The rotor is driven by a brushless rotary field motor. On sides of the carrier tube a coil is arranged which cooperates with a spoked-wheel magnet set into the radial rotor. A drawback of this blood pump is that the bearing stability in the radial direction is not optimal and that the pump because of the multiplicity of rotor and stator magnets has a high spatial requirement and high weight. In addition, the purely mechanical bearings give rise to wear in the axial direction.
The are also axial blood pumps. Here the journaling is effected exclusively mechanically in impeller wheels which are arranged at fixed locations in the carrier pipe ahead of and behind the rotor (Wernicke et al., A Fluid Dynamic Analysis Using Flow Visualization of the Baylor/NASA Implantable Axial Flow Blood Pump for Design Improvement, Artificial Organs 19(2), 1995, Pages 161-177). Such mechanical bearings are subject to wear and have an unsatisfactory effect on sensitive liquids, especially body liquids like blood.
An object of the invention is to provide a measuring device of the type described at the outset which has a substantially higher bearing stiffness, especially in the radial direction, and which therefore has numerous applications.
This object is achieved by the following features in accordance with the invention:
a) the measuring device has a carrier tube;
b) a rotor is journaled rotatably in the carrier tube;
c) the rotor is configured for interaction with the fluid found in the carrier tube;
d) the rotor has at both ends axially magnetized permanent magnet rotor magnets;
e) the ends of the rotor are juxtaposed with axially opposite permanently magnetic stator magnets connected to the carrier tube;
f) each stator magnet has such axial magnetism that it attracts the neighboring stator and rotor magnets; and
g) the measuring device has a magnetic axial stabilizing device for the rotor.
Alternatively, the object is achieved by a measuring device with the following features:
a) the measuring device has a carrier tube;
b) a rotor is rotatably journaled in the carrier tube;
c) the rotor is configured for the interaction with the fluid found in the carrier tube;
d) at the ends of the rotor there are respective axially magnetized permanently magnetic magnets opposite a flux conductive member whereby the magnets are either mounted on the rotor as rotor magnets or are connected to the carrier tube as stator magnets; and
e) the measuring device has a magnetic axial stabilizer unit.
The basic concept of the invention is thus, by means of rotor magnets and stator magnets arranged at the ends of the rotor, to generate a magnetic field in the axial direction respectively bridging the gaps between the rotor and stator magnets which respectively draws the opposing pairs of rotor and stator magnets in opposite directions toward one another. As a result, a bearing stiffness with the same geometry with respect to the magnet bearing of DE-A-29 19 236 can be obtained, but increased by at least one power of ten without thereby significantly affecting the annular passage between the carrier tube and the rotor hub. The aforedescribed effect also can be obtained when each magnet assembly has a magnet on one side and a flux-conducting device on the other side, as opposed to a system where each magnet assembly consists of two magnets, such as the rotor and stator magnets, which are juxtaposed. Thus, as an alternative, the magnet can be a rotor magnet mounted on the rotor and the flux-conducting member can be connected to the carrier tube or the flux-conducting member can be arranged on the rotor and the magnet fixed as a stator magnet on the carrier tube. For generating a high bearing stiffness, additional electric magnet coils can be provided for amplifying the magnetization of the flux-conducting member in the sense of increasing the attractive force between the magnets and the flux-conducting members.
To the extent that rotor and stator magnets pairwise are disposed opposite one another, they should preferably be composed of at least two interfitted partial magnets whereby respectively radially neighboring partial magnets are oppositely magnetized. Through this configuration of the rotor and stator magnets, a further increase in the bearing stiffness by a factor of 40 can be produced.
The rotor is preferably formed as an axial rotor so that there is an axial flow in the support tube. Such an axial rotor is substantially less expensive to shape than a radial rotor.
According to the invention, further, the rotor is provided with a rotor hub and the rotor magnets or flux guiding pieces are arranged in the rotor hub, whereby stator magnets or the flux-conducting pieces are disposed opposite the end faces of the rotor hub. The stator magnets or flux-conducting pieces can be connected with the carrier tube via ribs configured to promote flow. With this arrangement, a compact construction results which avoids to the greatest possible extent detrimental cracks. The stator magnets or flux-conductive pieces should be disposed in radial stabilizers whose contours do not project beyond the rotor hub whereby the radial stabilizers preferably have the same contours as the rotor hub.
In a further feature of the invention, at least one of the respective oppositely lying end faces of the radial stabilizers and the rotor can be of spherical configuration. With this configuration, it is possible to avoid, upon axial shifting of the rotor, a mechanical contact of axially spaced portions of the rotor and radial stabilizer. To limit the radial and/or axial mobility of the rotor it is advantageous to provide the respective oppositely lying end faces of the radial stabilizer and the rotor with mutually interdigitating complementary bearing pins and pin recesses, whereby a corresponding radial play is ensured such that bearing pins and bearing recesses will come into contact only upon relatively large deflections of the rotor in the radial direction.
According to a further feature of the invention, the rotor magnets and the stator magnets or flux-conducting pieces are arranged directly opposite one another respectively to ensure the strongest possible magnetic field.
According to the invention it is further proposed that the axial stabilization device have at least one electromagnetic coil as well as a control unit with a sensor detecting the axial movement of the rotor, whereby the control unit so influences the electric current flux in the magnet coil or the magnet coils that the magnetic field of the magnet coils counteracts an axial movement of the rotor out of its setpoint position. Such an axial stabilization device in principle is already known from DE-A-29 19 236 and DE-A-24 44 099 and has been found to be effective. Advantageously, the axial stabilizer can have two magnet coils which are arranged in the region of the stator magnets and/or the end faces of the rotor, whereby the axial stabilization is especially effective.
Thus there are two possibilities for arrangement of the magnet coils, namely, one in such manner that they surround the carrier tube and the second in which they are provided in the radial stabilizer itself whereby, however, then facilities must be provided for leading the conductors to and from the magnet coils. Preferably the magnetizable flux-conducting pieces of the radial stabilizer should be in such a configuration and arrangement that the axial magnetic field generated by the reactor and stator magnets is superimposed in the gap between the end of the radial stabilizer and reactor with the magnetic field generated by the magnet coils in axial direction and thus in a manner counteracting an axial movement of the rotor out of its intended position. The magnet coils can be used as sensors themselves. The flux-conducting pieces are preferably arranged at the level of the magnetic coils.
Advantageously the rotor has a pulse generator and the carrier tube a pulse pickup, whereby the pulse generator produces pulses detected by the pulse pickup and representing the speed of the rotor. This can be achieved in a simple manner by forming the pulse generator as pulse magnets and the pulse pickup as a coil so that in the coil an electric current is induced upon rotation of the rotor.
In a further feature of the invention it is provided that the carrier tube has at the level of the rotor, a three-phase stator which can be fed with three-phase current and the rotor has a radially magnetized plurality of spoked wheel magnet poles. The result is a synchronous motor which by supply of the three-phase stator with three-phase current generates a rotary field which entrains the rotor so that a rotary movement is imposed on the rotor. The measuring device in this case has motor properties. Preferably the magnets on the pole spokes are four magnetic segments magnetized in four different radial directions to counteract wobbling of the rotating rotor resulting from magnetic field asymmetry in the region of the bearing gap between rotor and radial stabilizer. The rotary field stator should be connected with an electronic three-phase generator operating with load angle regulation. The desired regulation of the load angle can be effective to set and stabilize the rotary field and torque applied to the motor depending upon the amount and direction of adjustment of the load angle.
The rotor can be matched to the respective purpose. For example, the rotor can have vane-type projections when the measuring device according to the invention is to be used as a centrifugal or turbine measuring unit for flowthrough measurements. The surface of the rotor can, however, be configured to be smooth and especially cylindrical, to the extent that the measuring device is configured in the aforedescribed manner as a synchronous motor for measurement of the viscosity of gaseous or liquid media by determining the electrical power utilized by the synchronous motor for maintaining a certain rotor speed. The viscosity is substantially proportional to the friction work at the outer surface of the motor so that the friction work in turn represents a measurement of the viscosity of the medium surrounding the rotor.
According to a further feature of the invention, it is proposed that the outer surface of the rotor have at least one radial outwardly extending projection and that a sensor is provided which detects the axial position of the rotor and generates a signal proportional to the axial position. The projection, preferably in the form of an annular rib, enables the possibility of measurement of the axial flow velocity of gaseous or liquid medium in which the axial force transmitted via the projection to the rotor is detected through the corresponding axial position shift of the rotor, whereby the axial stabilizing device generates a corresponding signal. Both features can be combined to the extent that the measuring device is provided with a synchronous motor in the above-described manner. Then the axial shift of the rotor and the drive power supplied to the rotor can be detected through the use of the synchronous motor so that both the flow velocity and the viscosity of the flow can be measured at the same time.